1. Field of the Invention
The present invention relates to a printing method for increasing thermal printer quality, and more specifically, to a printing method for increasing thermal printer quality when printing a pixel at a gray level x on paper by a printer.
2. Description of the Prior Art
Photo printers are different from general printers. The major difference is that photo printers print images such as a photo pictures on paper with high picture quality. Please refer to FIG. 1 and FIG. 2. FIG. 1 is a diagram of a prior art photo printer 10. FIG. 2 is a simplified exploded view of the photo printer 10 shown in FIG. 1. As shown in FIG. 1, the photo printer 10 has a ribbon 14, a thermal print head 12, a ribbon driver 18, and a roller set 20. The ribbon 14 has a plurality of sections, and each section is used for storing a different colored dye. The thermal print head 12 is fixed inside the photo printer 10 for heating the color dyes so that the color dyes are transferred onto a photo paper 16. The ribbon driver 18 is used for moving the ribbon 14 back and forth so that the thermal print head 12 can transfer a specific colored dye stored on the ribbon 14 onto the corresponding photo paper 16. The roller set 20 is used for holding the photo paper 16 and moving the photo paper 16 along a predetermined direction. Therefore, the fixed thermal print head 12 is capable of printing a color image on the photo paper 16.
As shown in FIG. 2, the thermal print head 12 has a plurality of heaters 22 that are arranged linearly and spaced equally for heating the ribbon 14. The colored dye stored on the ribbon 14 is heated, and is transferred onto the photo paper 16. When the thermal print head 12 starts printing images, each heater 22 positioned on the thermal print head 12 heats the ribbon 14 so that a plurality of corresponding pixels X1 will form a line image Y1. The photo paper 16 driven by the roller set 20 is then moved along the predetermined direction at a predetermined speed. Another line image Y2 is then printed on the same photo paper 16 next to the line image Y1. Accordingly, a plurality of line images are printed on the same photo paper 16 to complete the printing operation.
As mentioned above, the total number of heaters 22 positioned on the thermal print head 12 determines the corresponding number of pixels X1 of each line image printed on the photo paper 16. Moreover, the color concentration, that is, the gray level of each pixel X1 printed on the photo paper 16 is determined by the corresponding heater 22, which has a specific duration of each heating operation, and the total number of heating cycles.
Please refer to FIG. 3 and FIG. 4. FIG. 3 is a diagram of gray levels and a corresponding driving signal 30 according to the photo printer 10 shown in FIG. 1. FIG. 4 is a diagram of a binary data sequence of the driving signal 30 shown in FIG. 3. As shown in FIG. 3 and FIG. 4, before the thermal print head 12 of the photo printer 10 starts printing images onto the photo paper 16, all of the heaters 22 positioned on the thermal print head 12 are activated for a predetermined period Tp. In this way, each heater 22 will approach a predetermined printing temperature before printing. The above-mentioned procedure is called a preheating operation. In addition, the driving signal having a pulse with a binary value xe2x80x9c1xe2x80x9d activates the corresponding heater 22, and the driving signal corresponding to a binary value xe2x80x9c0xe2x80x9d deactivates the heater 22. Next, the photo printer 10 continuously activates the same heater 22 according to the corresponding gray level of the pixel X1. In other words, each heater 22 positioned on the thermal print head 12 is activated repeatedly according to the desired gray level of the corresponding pixel. The overall heating operation of the heater 22 is represented by a driving signal 30 and its corresponding binary values. The duration Tu of a pulse 32 is the heating time unit for activating the heater 22.
The heater 22 of the photo printer 10 can produce 256 (0xcx9c255) gray levels to print the corresponding pixel X1 with an appropriate gray level. A gray level corresponding to a lightest color concentration is equal to 0, and a gray level corresponding to a darkest color concentration is equal to 255. In other words, when the pixel X1 acquires a corresponding gray level equaling N, which is an integer between 0 and 255, the corresponding heater 22 is successively activated N times. Therefore, N pulses 32 of the driving signal 30 are repeatedly generated. That is, N binary xe2x80x9c1xe2x80x9d values are input to the heater 22 continuously. Please note that the photo paper 16 is printed one line at a time. Because each pixel X1 positioned on the same line may have different gray levels, each heater 22 has to wait for 255 durations Tu so that the thermal print head 12 can then print the next line image. That is, a first heater 22 could finish printing a corresponding pixel X1 with a smaller gray level within a short time. However, another heater 22 printing a corresponding pixel X1 with a greater gray level may take a longer time. The actual heating durations are therefore centralized in the early period of the total heating duration. The more continuous printing durations, the more the heat accumulation. Additionally, the heat accumulation effect causes the system temperature to increase and the next printing gray level will stray from the predetermined gray level thereby affecting the printing quality. For example when the pixel X1 acquires a corresponding gray level equaling 64, the corresponding heater 22 has to be successively activated 64 times. Therefore, 64 pulses 32 of the driving signal 30 are repeatedly generated. That is, 64 binary xe2x80x9c1xe2x80x9d values are input to the heater 22 continuously. In actuality, the heat accumulation effect usually makes the corresponding gray level greater than 64 and results in printing distortion.
It is therefore a primary objective of the present invention to provide a method of increasing thermal printer quality when printing a pixel at a gray level x on paper by a printer to solve the problems mentioned above.
Briefly summarized, a printing method is disclosed for printing a pixel at a gray level x on paper by a printer. The printer comprises a thermal print head, which comprises a heater for heating a ribbon to print pixels from a gray level 1 to mxe2x88x921 on the paper. The method comprises: if x is not greater than a value n, heating the ribbon x times and evenly distributing the heating initiation times of the x times between the time point 0 and the time point (m*(xxe2x88x921)/n) for printing the pixel at the gray level x on paper. If x is greater than a value n, the ribbon is heated x times and the heating initiation times of the n times are evenly distributed between the time point 0 and the time point (m*(nxe2x88x921)/n) and the heating initiation times of the xxe2x88x92n times is evenly distributed after the heating initiation time points of the n times.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.